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Magnetic Phase Transition and Relaxation Effects in LiFePO4

We report the observation of para - antiferromagnetic transition at ~ 50 K in lithium iron phosphate, LiFePO4 through DC magnetization and Mössbauer spectroscopy. The Ferrous ion Fe2+ (3d6, 5D) in LiFePO4 exhibits relaxation effects with a relaxation frequency ~1.076 \times 10(rise to 7) s-1 at 300 K. The temperature dependence of the frequency suggests the origin of the relaxation is spin-lattice type. The quadrupole splitting at low temperatures indicates the excited orbital states mix strongly to the orbital doublet ground state via spin-orbit coupling. Modified molecular field model analysis yields a saturation value for hyperfine field ~125 kOe. The anomaly in magnetization and Mössbauer parameters below 27 K may be ascribed to contribution of orbital angular momentum. The high value of the asymmetry parameter (η ~ 0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighbourhood for the ferrous sites.